For most anglers who have encountered both winter and summer steelhead, it’s abundantly clear there are differences between the two races of fish.
For one, they enter freshwater at different times of the year, although there is some overlap.
In addition, there are physical differences. Summer steelhead tend to be more fusiform — sleeker and more firm bodied — than their winter cousins. This is because summers are sexually immature and do not have fully formed gonads at the time of entry into freshwater. They also migrate further from the ocean and occupy different habitats — a more streamlined body likely helps them do this as it may allow for more efficient swimming and leaping.
Winters on the other hand, despite being larger than summers, on average, throughout much of their range, enter rivers sexually mature and have large bellies swollen with eggs and milt, which sometimes will be laid in the gravel within days of entering freshwater.
But there are other differences which are less apparent. A study by James Lamperth and Mara Zimmerman of WDFW and Tom Quinn at University of Washington looked into those differences. Specifically, they wanted to analyze whether summer and winter run steelhead had significantly different fat content. They tested for somatic lipid content in body tissues, which are fat stores that enable steelhead to survive in freshwater with little to no foraging.
The found, as expected, significantly different levels of somatic lipids. Summer steelhead had nearly twice as much fat as winter steelhead! This makes sense given that summer steelhead generally migrate much greater distances than winters and may spend up to a year in freshwater before spawning. They need those fat reserves to survive and to grow eggs and sperm. Winters, on the other hand, in most cases have already converted their fat into eggs and sperm by the time they enter streams from the salt.
Interestingly, the authors also found that fat content differed within the races themselves. For example, early-entering summers had a higher fat content than fish entering later. Makes sense, right? Earlier entry means a longer period of time in freshwater with minimal to no feeding before the animal is ready and able to spawn.
Further, Lamperth et al found that wild female summer steelhead had a 19% higher fat content than their male counterparts. This is likely due to the fact that there is a higher energy demand for producing eggs than sperm, and since these fish mature in freshwater they need these fat stores to develop their gonads.
Summer steelhead are generally more fusiform than their winter counterparts
The study also found that male winter steelhead had 27% more somatic lipids than winter females. This extra fat likely helps them stay in freshwater for up to a couple of months and spawn with multiple females. Males can also continue to produce sperm in freshwater, so more fat means more sperm. Spending more time in freshwater for male winter steelhead also equates to a higher likelihood of spawning success. So males with a higher fat content may be more likely to father more offspring, increasing their reproductive fitness. It is also one reason that male winter steelhead (from a hatchery, of course, as wild fish can’t be harvested in many rivers) tend to be much better table fare than female winters.
As any good griller knows, more fat = better bbq.
What about the relative fat content of hatchery versus wild fish? The Lamperth study, somewhat surprisingly, found that hatchery summer steelhead had 21% fewer somatic lipids than wild fish. This means they have less energy to put into survival and gonad development. Since hatchery fish are collected and often held in raceways for weeks to months before spawning it is less critical for these fish to have adequate fat stores to survive and complete the spawning cycle. Basically, fish which would be the most fit in the natural environment are no longer favored because of artificial selection in the hatchery.
Lower fat stores and the consequent reduction in likelihood of survival for adults and their offspring is one way that the fitness of hatchery reared fish may be reduced.
Lastly, the authors measured stable isotopes, which leave a chemical signature behind that allowed them to determine where steelhead were feeding in the ocean. They found that both winter and summer steelhead went to a similar part of the ocean and ate the same food. Despite a similar diet, however, hatchery summers were unable to store as much energy as wild summers, most likely due to artificial selection in the hatchery which does not favor fish with more fat.
Ultimately, this study highlights the fact that wild summer steelhead are really good at storing fat for surviving and completing their sexual maturation over long periods in freshwater, while winters use their fat in the ocean to mature.
This male winter steelhead has spent significant time in freshwater as evidenced by his coloration and scarring.
These results raise an important question, however: with climate change and other factors causing water temperatures to rise in many watersheds, will summer steelhead have to store even greater levels of fat to survive in the future? Warmer water temperatures elevate the metabolism of fish, causing faster conversion of fat into energy and potentially leading to individual summer steelhead dying because they simply don’t have enough energy to persist for as long as they need, and have evolved to do, in freshwater.
As we have discussed in previous Science Friday posts, wild steelhead are remarkably adaptive and have exceptionally diverse life histories, so they may well have solutions to these challenges encoded in their genes. Nonetheless, Wild Steelheaders United will continue to work on restoring and improving freshwater habitat conditions, to give these fish — and the fisheries they support — the best chance to persist.